Many people have sought for a long time to modify the colour of their hair and in particular to bleach it or, in contrast, to dye it, for example in order to conceal their white hair.
Essentially two types of dyeing operations have been developed in order to dye human keratinous fibres.
The first type of dyeing operation is “permanent” or “oxidation” dyeing, which employs dyeing compositions comprising oxidation dye precursors, generally known as oxidation bases. These oxidation bases are colourless or weakly coloured compounds which, in combination with oxidizing products, can give rise, by an oxidative condensation process, to coloured compounds.
It is also known that it is possible to vary the shades obtained with these oxidation bases by combining them with couplers or colouring modifiers. The variety of the molecules involved as oxidation bases and couplers makes it possible to obtain a rich palette of colours.
The second type of dyeing operation is “semipermanent” dyeing or direct dyeing, which consists in applying direct dyes to the keratinous fibres, direct dyes being coloured and colouring molecules having an affinity for the said fibres, in leaving to stand and in then rinsing them.
In order to carry out these dyeing operations, the direct dyes generally employed are chosen from nitrobenzene, anthraquinone, nitropyridine, azo, xanthene, acridine, azine or triarylmethane direct dyes.
This type of process does not require the use of an oxidizing agent in order to develop the colouring. However, it is possible to employ such an agent in order to obtain a lightening effect with the dyeing operation. Reference is then made to a direct or semipermanent dyeing operation under lightening conditions.
Permanent or also semipermanent dyeing methods under lightening conditions thus consist in employing, with the dyeing composition, an aqueous composition comprising at least one oxidizing agent, under alkaline pH conditions in the great majority of cases.
The role of this oxidizing agent is to decompose the melanin of the hair, which, depending on the nature of the oxidizing agent present, results in a more or less pronounced lightening of the fibres. Thus, for a relatively slight lightening, the oxidizing agent is generally hydrogen peroxide. When greater lightening is desired, use is normally made of peroxygenated salts, such as, for example, persulphates, in the presence of hydrogen peroxide.
In order to improve the performances of the methods for dyeing and/or bleaching human keratinous fibres and to limit the inconveniences related to the use of alkaline agents and of oxidizing agents, the proposal has been made to employ, in the dyeing compositions, a substantial amount of one or more fatty substances.
However, the incorporation in these compositions of a sufficient amount of fatty substance proves to be problematic, all the more so when the dyeing composition is formulated in the form of a conventional emulsion of oil-in-water type. This is because these formulations are thermodynamically unstable and their viscosity changes over time, all the more so if the amount of fatty substance is high.